In the processing of petroleum hydrocarbons and feedstocks such as crude oil and petroleum processing intermediates, and petrochemicals and petrochemical intermediates, e.g., gas, oils and reformer stocks, chlorinated hydrocarbons and olefin plant fluids such as deethanizer bottoms the hydrocarbons are commonly heated to temperatures of 100.degree. to 1000.degree. F. Similarly, such petroleum hydrocarbons are frequently employed as heating mediums on the "hot side" of heating and heat exchange systems such as vacuum tower bottoms and slurry systems. In both instances, the petroleum hydrocarbon liquids are subjected to elevated temperatures which produce a separate phase known as fouling deposits, within the petroleum hydrocarbon. In all cases, these deposits are undesirable by-products. In many processes, the deposits reduce the bore of conduits and vessels to impede process throughput, impair thermal transfer, and clog filter screens, valves and traps. In the case of heat exchange systems, the deposits form an insulating layer upon the available surfaces to restrict heat transfer and necessitate frequent shut-downs for cleaning. Moreover these deposits reduce throughput, which of course, results in a loss of capacity with a drastic effect in the yield of finished product. Accordingly, these deposits have caused considerable concern to the industry.
Organic foulants are usually higher molecular weight materials ranging in consistency from that of tar to rubber to "popcorn" to "coke". The exact composition of such foulants is difficult to identify.
One particularly troublesome type of organic fouling is caused by the formation of polymers that are insoluble in the hydrocarbon or petrochemical fluid being processed. The polymers are usually formed by reactions of unsaturated hydrocarbons, although any hydrocarbon can polymerize. Generally, olefins tend to polymerize more readily than aromatics, which in turn polymerize more readily than paraffins. Trace organic materials containing hetero atoms such as nitrogen, oxygen and sulfur also contribute to polymerization.
Polymers are formed by free radical chain reactions. These reactions, shown below, consist of two phases, an initiation phase and a propagation phase. In reaction 1, the chain initiation reaction, a free radical represented by R.sup..cndot., is formed (the symbol R can be any hydrocarbon). These free radicals, which have an odd electron, act as chain carriers. During chain propagation, additional free radicals are formed and the hydrocarbon molecules (R) grow larger and larger (see reaction 2c), forming the unwanted polymers which accumulate on heat transfer surfaces.
Chain reactions can be triggered in several ways. In reaction 1, heat starts the chain. Example: when a reactive molecule such as an olefin or a diolefin is heated, a free radical is produced. Another way a chain reaction starts is shown in reaction 3. Here metal ions initiate free radical formation. Accelerating polymerization by oxygen and metals can be seen by reviewing reactions 2 and 3.
1. Chain Initiation PA1 R--H.fwdarw.R.sup..cndot. +H PA1 a. R.sup..cndot. +O.sub.2 .fwdarw.R--O--O.sup..cndot. PA1 b. R--O--O.sup..cndot. +R'--H.fwdarw.R'.sup..cndot. +R--O--O--H PA1 c. R.sup..cndot. +C.dbd.C.fwdarw.R--C--C.sup..cndot. .fwdarw.polymer PA1 a. Me.sup.++ +RH.fwdarw.Me.sup.+ +R.sup..cndot. +H.sup.+ PA1 b. Me.sup.++ +R--O--O--H.fwdarw.Me.sup.+ +R--O--O.sup..cndot. +H.sup.+ PA1 a. R.sup..cndot. +R'.sup..cndot. .fwdarw.R--R' PA1 b. R.sup..cndot. +R--O--O.sup..cndot. .fwdarw.R--O--O--R PA1 330 g. of polyisobutenylthiiophosphonic acid (0.1 mole), (MW of isobutenyl moiety.apprxeq.1300) 11.8 g. of hexylene glycol (0.1 mole) and 100 g. of xylene were added to a 500 mL reaction kettle equipped with thermometer, traps, condenser and drying tube. The mixture was slowly heated to reflux (.apprxeq.150.degree. C.) and maintained for about two hours. After this, the temperature was slowly increased. Between about 138.degree.-176.degree. C., liquids starting to condense in the traps leaving hexylene glycol ester of polyisobutenyltiophosphonic acid (HGETPA) in the flask. This product was analyzed for residual alcohol and none was detected. PA1 330 g. of polyisobutenylthiophosphonic acid (0.1 mole), (MW of isobutenyl moiety.apprxeq.1300) 7.4 g. n-butanol (0.1 mole) and 100 g. xylene were added to a 500 mL reaction kettle equipped with thermometer, traps, ice condenser, and drying tube. PA1 The mixture was heated to reflux slowly over about a two hour period. Condensates were caught in the trap, with the resulting n-butanol ester of polyisobutenyltiophosphonic acid (BETPA) remaining in the reaction kettle. This product was analyzed for residual alcohol and none was detected. PA1 (1) phenylenediamine compounds such as N-phenyl-N'(1,3-dimethylbutyl)-p-phenylenediamine, N-phenyl-N'(1,4-dimethylpentyl)-p-phenylenediamine, or N-phenyl-N'(1, 4-dimethylpropyl)-p-phenylenediamine; PA1 (2) phenolics such as ortho-tert-butyl-para-methoxyphenol, cresylic acid, aminophenol, 2,6-ditertiarybutylphenol, or 4,4' methylenebis-(2,6-ditertiarybutylphenol); PA1 (3-quinones such as tertiary butyl catechol, benzoquinone, tetrabutyl hydroquinone and the like; PA1 (4) alkaline earth salts of alkylphenol sulfides, such as calcium or magnesium sulfurixed phenates; PA1 (5) sulfur/amine containing materials such as phenothiazine and alkylated derivatives or sulfur/phosphorus containing materials such as metal or amine salts of dialkyl dithiophosphoric acids. PA1 (1) substituted amines such as tetrahydropyrimidene, imidazolines, alkylene polyamines and the like; PA1 (2) corrosion inhibiting reaction products obtained by a) reacting at least one alkylene polyamine with a sufficient quantity of at least one aliphatic carboxylic acid to produce a salt of said amine and acid, said salt being of such nature that the amine reactant is decharacterized to the extent that the likelihood of an amine--aldehyde condensation polymerization is substantially eliminated and (b) reacting the salt with a lower aldehyde. See U.S. Pat. No. 3,567,623-especially preferred is the reaction product of CH.sub.3 (CH.sub.2).sub.17 --NH--(CH.sub.2).sub.3 --NH.sub.2, a tall oil head, and paraformaldehyde--see Example 1 of U.S. Pat. No. 3,567,623; PA1 (3) alkaline earth (Group 2) metal salts of oil-soluble alkyl benzene sulfonic acids, such as magnesium or calcium sulfonates; PA1 (4) amine salts of oil-soluble alkyl naphthalene sulfonic acids, such as the ammonium or ethylenediamine sulfonates; PA1 (5) 2,5-dimercapto-1,3,4-thiadiazole and derivatives; PA1 (6) ethoxylated or propoxylated derivatives of alkyl phenols. PA1 (1) N,N'-disalicylidene-1,2-cyclohexanediamine; PA1 (2) sodium N,N'-ethylenebis(2,5-sodium sulfocarbolate) glycinate; PA1 (3) 2,5-dimercapto-1,3,4-thiadiazole derivatives; PA1 (4) reaction products of alkylphenol, aldehyde, and polyamine such as nonylphenol, formaldehyde and ethylenediamine; optionally, dialkyl or alkoxy phenols may be used.
2. Chain Propagation
3. Chain Initiation
4. Chain Termination
Research indicates that even very small amounts of oxygen can cause or accelerate polymerization. Accordingly, to inhibit this insidious fouling problem, it is highly desirable to provide a polyfunctional process antifoulant which can, among other functions, inhibit oxygen based polymerization initiation. This antioxidant function serves as a "chain-stopper" by forming inert molecules with the oxidized free radical hydrocarbons, in accordance with the following reaction: